This invention relates to the field of well logging and, more particularly, to well logging techniques and apparatus for determining formation resistivity at different radial depths of investigation and with greater accuracy than prior techniques, for determining the existence, locations and properties of beds and caves, and for determining changes in the size of an earth borehole. The invention has general application in the well logging art, but is particularly useful in measuring while drilling.
A commonly used technique for evaluating formations surrounding an earth borehole is resistivity logging. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while porous formations with low resistivity are generally water saturated. However, the region immediately surrounding the borehole can be invaded by borehole fluid or mud filtrate and have a different resistivity than the virgin formation. If a resistivity logging device has only one radial depth of investigation, there is limited ability to measure resistivity of all zones of interest, and there may be difficulty in determining if the measured resistivity represents the invaded zone, the virgin zone, or some combination of the two zones. However, if the resistivity logging device has at least two radial depths of investigation, there is much greater flexibility. In addition to the advantage of having, for example, a shallow measurement and a deep measurement individually, the combination of the two provides additional information such as the extent of invasion. It is also possible to use the shallow reading to correct the deeper reading and thereby obtain a better estimate of the true formation resistivity.
Therefore, wireline resistivity logging tools usually are provided with two or more radial depths of investigation. Common wireline resistivity logging tools achieve two depths of investigation by using a short and a long vertical array of electrodes or coils. In general, a long vertical array provides a greater radial depth of investigation than does a short vertical array.
A type of well logging which is of interest herein is so-called electromagnetic propagation logging, which can be used to measure the resistivity of the formation surrounding a borehole. For example, U.S. Pat. No. 3,551,797 describes a technique wherein electromagnetic energy is transmitted into the formation, and energy shed back into the borehole is measured at a receiver pair to determine the attenuation and/or the phase shift of the electromagnetic energy propagating in the formation. (In this type of device, a receiver pair is generally used in so-called "differential receiver" arrangement to facilitate the accurate measurement of attenuation and/or phase shift of the formations.) This patent teaches that by using more than one vertical spacing between a transmitter and different receiver pairs, different radial depths of investigation can be attained. For example, a relatively close (to the transmitter) receiver pair can be utilized to obtain attenuation and/or phase information from which the properties of the invaded zone are determined, and measurements of attenuation and/or phase from a relatively far (from the transmitter) pair of receivers can be utilized to obtain the properties of the deeper virgin formations. In this U.S. Pat. No. 3,551,797, the concern is largely with obtaining conductivity. Either attenuation or phase shift can be utilized therein to determine the skin depth for the formation, with the conductivity then being determinable from the skin depth. Below a certain frequency range, the skin depth of the electromagnetic energy can be calculated using either attenuation or phase information, since displacement currents have minimal effect.
Various other techniques exist in the art for utilizing extra receivers to investigate resistivity at different depths of investigation. For example, in U.S. Pat. Nos. 4,451,789 and 4,107,597 three receiver coils are used in obtaining different radial depths of investigation.
A technique also of interest as background herein is disclosed in U.S. Pat. No. 4,209,747, which describes an electromagnetic method and apparatus for determining the dielectric constant and/or the conductivity of formations. This patent observes that when electromagnetic energy is emitted from a first location in a borehole, the volume and shape of the formations which affect a measurement of wave energy attenuation as measured at the second location is different than the volume and shape of the formations which affect a measurement of relative phase of the electromagnetic energy received at the second location. In particular, the attenuation measurement is a deeper measurement than the phase shift measurement. This principle is referred to and exploited in a form of the present invention. The '747 patent describes a method and apparatus in which means are provided for generating electromagnetic energy at a first location in the borehole. Further means are provided for detecting attenuation at a second location in the borehole, and further means are provided for measuring phase shift at a third location in the borehole. The third location is farther from the transmitter than the second location. The attenuation and phase shift are measured at each location by differential receiver pairs, so there are four receivers used. The attenuation measured at the second location and the phase shift measured at the third location have substantially the same depth of investigation. Therefore, these two quantities are used to determine the dielectric constant and/or conductivity for the same regions of the formation. The '747 patent is largely concerned with improved accuracy in the determination of dielectric constant and/or conductivity, and a preferred frequency for the technique is 20 MHz, where displacement currents are substantial, so that dielectric constant is a measurable quantity in formations typically encountered in well logging.
While it is very advantageous to have measurements of formation resistivity at different depths of investigation, prior art techniques which require different receiver spacings to achieve this end generally suffer one or more of the following drawbacks: the need for additional receivers, increase in the length of the logging device to accommodate the different spacings; additional circuitry and wiring; higher cost. These drawbacks can be significant in wireline logging, but tend to be even more serious in measurement while drilling systems, wherein the measuring environment is particularly hostile, and stricter requirements are usually imposed on the acceptable length and the mechanical strength of the logging apparatus. (Examples of electromagnetic logging devices used in measuring while drilling are set forth in British Published Application No. 2,146,126 and U.S. Pat. No. 4,553,097. In these patents, a transmitter and a receiver pair are mounted recessed in a drill collar, and amplitude and/or phase measurements are taken. One depth of investigation is obtained.) To minimize the possibility of mechanical damage, and to maintain the mechanical strength of the drill string, the number of sensors and the total length of the sensors should be minimized.
It is among the objects of the present invention to provide an improved apparatus and method for determining formation resistivity at different depths of investigation. It is also among the objects hereof to devise such an apparatus and method and have it be suitable and advantageous for use in measurement while drilling.
In the prior art, so-called "borehole compensation" has been used to advantage in certain types of logging devices. For example, the technique has been commonly employed in acoustic logging, has been utilized in electromagnetic logging pad-mounted devices (see e.g. U.S. Pat. No. 3,849,721), and has also been suggested for use in a centralized electromagnetic logging device using transverse magnetic mode components of the electromagnetic energy (see U.S. Pat. No. 4,553,097). In a typical borehole compensated logging device, a pair of receivers are located between two transmitters. The transmitters are alternately energized, and the signals received at the receivers can be processed (generally, averaged) to reduce or eliminate the effects of factors such as the logging device being eccentered in the borehole and irregularities of the borehole or mudcake. In various well logging applications, borehole compensation has apparently not been considered practical. The need for transmitters on both sides of the receiver pair means that the logging device must be substantially longer than the length of the same device if it had only a single transmitter. For logging devices which have a relatively long transmitter-to-receiver spacing to begin with, a further transmitter on the other side of the receivers may render the device longer and more cumbersome than is acceptable. A further problem which can arise as a consequence of having transmitters on both sides of the device receivers is the additional noise encountered at the receivers as a result of having a second transmitter. One source of this noise is the cross talk from the wires which carry high power energizing signals to the second transmitter. Assuming that the energizing signals for both transmitters come from a common source, then the wiring coupling the energizing source to one of the transmitters must run past the receivers. Since the energizing signals are much larger than the received signals, the signals at the receivers are particularly susceptible to contamination by noise from high amplitude transmitter signals passing nearby. Furthermore, in logging devices which have relatively long transmitter-to-receiver spacings, the signals received at the receivers are relatively weak (as compared to the same device if it had shorter spacings), and the problem of cross-talk from the transmitter wiring is exacerbated.
For the reasons just set forth, among others, it would be expected that borehole compensation in an electromagnetic propagation logging device utilized for logging-while-drilling would be particularly problematic. In addition to the difficulties already described, the noise and other instabilities caused by the drilling environment can render the task even more difficult. Further, considerations of the length of the logging-while-drilling apparatus (including the factor of mechanical strength) are not conducive to the use of features that typically tend to require increased device length; namely, provision for different depths of investigation, and employment of borehole compensation.
It is among the objects of the present invention to provide a practical borehole compensation capability for a mandrel type of electromagnetic propagation logging system, and for such a system as utilized for logging while drilling.
There are various techniques in the well logging art for determining the presence, conductivity, and location of beds in formations surrounding a borehole and for determining the location and size of borehole caves. Generally, these techniques can stand improvement in one or more of the following aspects: difficulty in recognizing the presence of a thin bed; difficulty in determining the conductivity of a thin bed; confusion of a thin bed with a cave; difficulty in obtaining the location of the boundaries of beds and caves with good accuracy; complexity of the equipment and/or processing techniques needed to obtain the desired information. It is among the objects of the invention to provide improvement in these areas, and to provide improvements which are applicable for use in measurement while drilling.
Using a mechanical caliper to measure the diameter of an oil well borehole is a well known technique in wireline logging. A borehole caliper log contains useful information about the subsurface formations. For example, the borehole diameter can be used to differentiate between shales and sandstones since boreholes tend to wash-out in the shales whereas boreholes tend to remain at the bit size in the sandstones. A caliper log can be used as an aid in interpreting other logs which are sensitive to the borehole diameter. A caliper log is also used to compute the amount of cement required for well casing. It can also be used to aid a drilling program by identifying sections of the well where the drill string might stick and by evaluating the effectiveness of the mud program in preventing swelling or caving of the borehole.
Wireline caliper tools generally use moveable arms which expand and contact the borehole wall. Typically, the caliper tool is lowered into the borehole on a wireline cable with the moveable arms in a retracted position so that they do not contact the borehole wall. The arms are engaged with the borehole wall when the tool is pulled out of the well by the wireline cable. Application of moveable arm calipers to measurement while drilling would be very difficult because of the very harsh mechanical environment.
It is among the objects of the present invention to provide an improved technique for measuring changes in borehole diameter, and to provide improvements which are applicable to measurement while drilling.